Implantable pacemakers generate electrical stimulation pulses and deliver such stimulation pulses to atrial and/or ventricular muscle tissue of a patient's heart at a prescribed rate when, through disease or other causes, the heart is not able to maintain the prescribed heart rate on its own. "Capture" of the patient's heart occurs when the applied electrical stimulus generated by the pacemaker is of sufficient energy to stimulate or depolarize the cardiac muscle tissue, thereby causing a cardiac contraction thereby forcing the heart rate or rhythm to track the delivery of the electrical stimuli. A capture event is typically represented by an evoked R-wave of an intercardiac electrogram (IEGM) signal. Capture fails to occur when the applied stimulus is of insufficient energy to stimulate or depolarize the cardiac tissue and the heart rate is not controlled by the pacemaker. This is also referred to as a "loss-of-capture" event. Needless to say, for a cardiac pacemaker to properly perform its intended function, it is critically important that the electrical stimuli it issues be of sufficient energy to capture the heart, i.e., to cause the cardiac tissue to depolarize.
The energy of the electrical stimuli generated by an implanted pacemaker is derived from the energy stored in the pacemaker battery. The pacemaker battery has a limited amount of energy stored therein, and the generation of electrical stimuli represents by far the greatest drain of such energy. In order to preserve this limited energy and prolong the life of the pacemaker battery, it is known in the art to adjust the energy of the delivered electrical stimuli so that it is just sufficient to cause capture, with an appropriate safety margin. See, e.g., U.S. Pat. Nos. 3,949,758 and 4,686,988. The amount of energy needed to effectuate capture is known as the capture "threshold", and electrical stimuli of energy less than the capture threshold do not bring about capture, while electrical stimuli of energy greater than the capture threshold do bring about capture. Many recent generation pacemakers also include autocapture and autothreshold features that enable the value of the capture threshold of a given patient to be regularly determined, thereby enabling the output energy of the pacemaker stimulus to be optimally set to an autocapture value that is above the capture threshold, but not too far above the capture threshold, so as to conserve the energy in the pacemaker battery.
Reliably determining capture within an implantable pacemaker is not an easy task. Some implantable pacemakers perform a Capture Verification Routine on a continuing basis and take remedial actions whenever a loss-of-capture occurs. Other Capture Verification Routines are performed by the implantable pacemaker on a periodic basis to automatically maintain the capture threshold at an appropriate value. It will be appreciated that these "autocapture" routines, therefore, are critical to maximize the effectiveness and efficiency of the pacemaker. Recent generation implantable pacemakers typically initiate their autocapture routine, if not on a regular basis, then when two or more successive loss-of-capture events occur. When this happens, the autocapture routine instructs the pacemaker to incrementally increase the energy of the output stimulus until capture occurs. As a safety precaution, a high output backup stimulus is provided, whenever there is a detected loss-of-capture, in order to maintain the cardiac rhythm of the patient.
Another feature commonly found in recent generation implantable pacemakers is an "autothreshold" routine which is periodically employed to ensure that the energy level of stimulation pulses are as low as possible in order to conserve the pacemaker's battery power, but high enough to safely insure capture. During an autothreshold routine the implantable pacemaker decrements the energy of its output stimulus until capture is lost. The output energy is then incrementally increased until capture is regained. As with the autocapture routine, every loss-of-capture of the primary output stimulus is typically followed by a high output back-up stimulus in order to maintain the cardiac rhythm of the patient during the process. Because of their similar functions, the autocapture and autothreshold routines of a pacemaker are usually conducted in conjunction or even simultaneously with each other. Thus, the term "autocapture/autothreshold" routine will be used herein to refer to either or both an autocapture routine or an autothreshold routine.
The presence of fusion events or fusion beats during operation of autocapture/autothreshold routines can introduce anomalies into the intercardiac electrogram (IEGM) data used by those routines to verify capture. A fusion beat is typically a ventricular or atrial depolarization that starts from two foci, one spontaneous and the other a result of pacemaker stimulus. Fusion beats can result in the magnification, diminishment or abolition of the R-wave of the sensed voltage signal. If magnified, the voltage signal could be misclassified as an evoked signal, resulting in the pacemaker missing a loss-of-capture event and defining the autocapture and/or autothreshold value as lower than it actually is. Conversely, if the R-wave of the sensed voltage signal is diminished or abolished by the fusion beat, the pacemaker may incorrectly identify the signal as a loss-of-capture, resulting in autocapture/autothreshold values being set higher than the actually are.
What is needed, therefore, is a very reliable method of discriminating between capture and loss-of-capture in an implantable pacemaker and, more particularly, a method of determining whether a response sensed during an autocapture/autothreshold routine is a fusion beat, which may or may not be indicative of capture, or is an evoked response. When a fusion beat is identified during the autocapture/autothreshold routines, the corresponding IECG data should be identified appropriately and the routines repeated to the extent necessary to make a clear determination of capture or loss-of-capture.
The classical approach to determine capture during the autocapture/autothreshold routines is to apply a ventricular stimulus (V-pulse) of varying output energy (i.e., amplitude and/or width) in order to search for the capture threshold. The implantable pacemaker looks for an evoked R-wave response with each and every V-pulse applied. The evoked R-wave response is typically monitored within 5-100 msec of the V-pulse (i.e., within a prescribed time window after the issued V-pulse, generally referred to as the "capture detection window") to determine if the issued V-pulse caused capture. If no ventricular evoked response is sensed, a subsequent ventricular "backup" pulse may be issued, and the search for the capture threshold continues. If a ventricular response is sensed, then the pacemaker circuits assume that the capture threshold is less than the energy of the most-recently issued V-pulse, and thereafter sets the V-pulse energy to an appropriate value above such threshold.
Several related art methods of capture verification are disclosed, for example, in U.S. Pat. Nos. 4,686,988, 4,817,605, and 5,222,493 (all of Sholder and each incorporated in its entirety by reference herein) which deal with sensing the evoked response using various sensing configurations and a special evoked response (ER) amplifier. None of these patents disclose methods of discriminating fusion beats nor do they recognize that capture can be determined from an evaluation and analysis of the T-wave and the V-pulse to T-wave time interval.
In U.S. Pat. Nos. 4,674,508; 4,674,509; 4,708,142; 4,729,376; and 4,913,146 (all of DeCote, Jr. and each incorporated in its entirety by reference herein), the autocapture and capture-determining methods involve issuing a double stimulation pulse, separated in time by less than the natural refractory period of the heart. Hence, at most only one of the pulses can induce cardiac capture. DeCote Jr. teaches subtracting the post-pulse lead recovery artifacts (50 msec following pulse) for both pulses. In the absence of capture, such post-pulse artifacts are essentially identical, and the difference is zero. If capture occurs, however, the post-pulse artifacts are significantly different, and the difference is not zero. Again, none of these patents disclose methods of discriminating fusion beats from the post-pulse artifacts nor do they recognize that capture can be determined from an evaluation and analysis of the T-wave characteristics.
Still other related art is disclosed in U.S. Pat. No. 5,350,410 issued to Kleks et al. and also incorporated in its entirety by reference herein. In the Kleks et al. disclosure the Capture Verification Routine also uses double pulses, but uses a more sophisticated technique than the aforementioned DeCote Jr. techniques for digitally analyzing and comparing the evoked response after the pulses. See also U.S. Pat. No. 5,766,229 (Bornzin) issued Jun. 16, 1998, incorporated in its entirety by reference herein, which teaches that capture should be verified only infrequently, and only when the heart rhythm is stable. The Bornzin disclosure describes a method of capture verification that involves shortening the cycle length significantly from that of the stable rhythm, which advantageously reduces the likelihood of fusion.
The T-wave is mostly ignored in prior art patents that rely on the IEGM signal for sensing when certain cardiac events occur. In many instances, the existence of the T-wave in the IEGM signal is regarded as a nuisance. A few related art patents, however, do rely on the T-wave for certain purposes. For example, U.S. Pat. No. 4,556,062 issued to Grassi et al., incorporated in its entirety by reference herein, discloses the use of the slope of the T-wave as an indication of what the basic stimulation frequency of a rate-responsive pacer should be.
In addition, U.S. Pat. No. 4,228,803, issued to Rickards and incorporated in its entirety by reference herein, teaches measuring the time interval between the stimulus pulse and the following T-wave. Variation in such stimulus to T-wave interval is then used as a parameter to control the pacemaker escape interval, thereby adjusting the pacing rate as a function of the stimulus to T-wave interval. There are also several related art references that utilize the mere sensing of the T-wave or T-wave parameters to aid in the verification of capture. These related art references include U.S. Pat. No. 5,161,529 issued to Stottes et al.; U.S. Pat. No. 4,880,004 issued to Baker Jr. et al.; and U.S. Pat. No. 4,557,266 issued to Schober; each of which is incorporated in its entirety by reference herein.
More recently, the T-wave has been used as part of a Capture Verification Routine as disclosed in U.S. Pat. No. 5,476,487 (Sholder), issued Dec. 19, 1995, the disclosure of which is incorporated by reference herein. The Sholder patent teaches a method of determining capture or lack-of-capture by measuring the stimulus-to-T-wave time period rather than looking for an evoked response immediately following application of the V-pulse. In particular, it looks for a significant change in the stimulus-to-T-wave time for a series of ventricular pulse pairs, where the first pulse of each pair has decreasing energy from that of a first pulse of a prior pair, and the backup pulse of each pair is always of sufficient energy to cause capture. The time interval between the first stimulus to the T-wave is compared to a reference value for a given patient. If there is a significant change in this time interval, then the first pulse is assumed to have lost capture and any resulting T-wave occurred due to the backup pulse. Thus, the Sholder patent teaches capture or loss-of-capture based on a change in the stimulus to T-wave interval.
None of these references or applications recognizes that, during an autocapture/autothreshold routine, variations in the T-wave morphology, pattern and/or stimulus to T-wave time interval may be used to determine whether a sensed voltage signal, occurring during the capture detection window, is an event signifying capture, signifying a fusion event or signifying loss-of-capture. Thus, the occurrence of fusion events during autocapture/autothreshold routines continues to result in inefficient and potentially dangerous adjustments to a pacemaker's operational parameters, as such fusion events are mistaken either for evoked responses to a stimulation pulse or for a loss-of-capture event.